1. Field of the Invention
The present invention relates to a multilayer mirror, an evaluation method, and an exposure apparatus.
2. Description of the Related Art
In manufacturing a fine semiconductor device using the photolithography technology, a conventional projection exposure apparatus projects and transfers a circuit pattern of a mask (reticle) onto a wafer via a projection optical system.
The minimum size (or resolution) transferable by the projection exposure apparatus is in proportion to the wavelength of the light used for the exposure, and inversely proportional to the numerical aperture (“NA”) of the projection optical system. Therefore, use of a shorter wavelength improves the resolution, and the use of a shorter wavelength of the exposure light is promoted along with the recent demand for fine processing to semiconductor devices. More specifically, use of a short wavelength of the ultraviolet (“UV”) light as the exposure light is promoted from the ultra-pressure mercury lamp (i-line with a wavelength of about 365 nm) to the KrF excimer laser (with a wavelength of about 248 nm and the ArF excimer laser (with a wavelength of about 193 nm).
However, the lithography using the UV light has limits in view of the recent increasingly miniaturized semiconductor devices. Accordingly, one proposed projection exposure apparatus uses the EUV light having a wavelength between about 10 nm and 15 nm that is shorter than a wavelength of the UV light, which will be referred to as a EUV exposure apparatus.
The light absorption in a material becomes conspicuous in the EUV region, and a refraction type optical element or dioptric system that utilizes refractions of the light used for the visual light and the UV light is no longer practical. Accordingly, the EUV exposure apparatus uses a reflection type optical element (catoptric system) that utilizes reflections of the light. The reflection type optical element in the EUV exposure apparatus includes a grazing-incidence total-reflection mirror and a multilayer mirror.
In the EUV region, a real part of the refractive index is slightly smaller than 1, and generates total reflection if the incident angle is made so large that the EUV light is incident close to the reflection surface. A grazing-incidence total-reflection mirror can usually maintain its reflectance of 80% or higher for obliquely incident light within scores of degrees from the reflection surface. Since the grazing-incidence total-reflection mirror has a small design freedom and leads to a large optical system, use of the mirror is impracticable.
Accordingly, the reflection type optical element in the EUV exposure apparatus uses a multilayer mirror that alternately layers two kinds of materials, such as molybdenum (Mo) and silicon (Si), having different optical constants. A sum of two layer thicknesses of two kinds of materials is generally referred to as a layer thickness.
The multilayer mirror can be used at an incident angle close to the normal incidence and maintains a high reflectance. The multilayer mirror reflects, when receiving the EUV light, the EUV light with a specific wavelength, exhibiting the wavelength selectivity. For example, where is an incident angle, is a wavelength of the EUV light, d is a layer thickness, and m is an order, the efficiently reflected EUV light is the one having a narrow bandwidth around the wavelength as the center which approximately satisfies the Bragg's equation as Equation 1 below:2 d×cos θ=m×λ  EQUATION 1
The EUV exposure apparatus is used to expose a circuit pattern of 0.1 μm or smaller, and requires a highly precise reflection surface shape for the multilayer mirror (particularly in the projection optical system). For example, a shape error budget σ (rms value) permitted to the mirror is given in the Marechal's criterion as Equation 2 below, where λ is a wavelength of the EUV light, and n is the number of multilayer mirrors in the projection optical system. For instance, when the projection optical system has four multilayer mirrors, and the wavelength λ of the EUV light is 13 nm, the shape error budget σ is 0.23 nm.
                    σ        =                  λ                      28            ×                          n                                                          EQUATION        ⁢                                  ⁢        2            
The Mo/Si multilayer film as the multilayer film for the multilayer mirror in the EUV exposure apparatus generally has a stress, which deforms a multilayer-mirror substrate and causes fluctuations in the wavefront of the reflected light (or the reflected wavefront). As a result, the optical characteristic, such as an imaging characteristic, of the multilayer mirror deteriorates.
There are conventionally proposed some technologies that reduce a deformation of the multilayer-film substrate caused by the multilayer film. See U.S. Pat. No. 6,134,049. This reference restrains a deformation of a substrate ST, as shown in FIG. 13, by arranging between the substrate ST and a multilayer film (reflection layer) RLL a multilayer film (stress compensation layer) SRL that produces an (inverse) stress opposite to the stress by the reflection layer RLL that reflects the EUV light. FIG. 13 is a schematic sectional view showing a structure of the conventional multilayer mirror having the stress compensation layer SRL.
Nevertheless, the stress compensation layer that does not maintain a desired precision cannot cancel out the stress of the reflection layer, leaving the deformation of the substrate, and further deforming the substrate. In addition, even for the substrate ST that maintains at the desired shape, the stress compensation layer SRL that has an uneven layer thickness distribution, as shown in FIG. 14, disturbs reflected wavefront RWS from the reflection layer RLL. It is important for a high imaging characteristic to precisely form the stress compensation layer on the substrate, and there is a demand to examine or evaluate the stress compensation layer. However, the stress compensation layer is formed between the substrate and the reflection layer, and it is very difficult to examine the stress compensation layer in a non-destructive manner.
In addition, an evaluation of the reflected wavefront of a single multilayer mirror is very difficult, and an evaluation method that can easily evaluate the multilayer mirror is sought.